Error free transmission of information is important in the communications industry today. In order to effectively transmit information modulated on radio frequency signals either over the air or on a medium such as a coaxial cable, it is helpful to properly set the power level at which modulated radio frequency signals are to be transmitted. The effective use of transmission power is the focus of much attention. Communications engineers are frequently concerned with is maximizing the amount of power used to transmit signals without causing distortion that will cause errors in the demodulation of data on the receiving side of the communications channel. Another concern is setting the desired amount of transmit power to the level necessary to ensure that the intended receivers receive the signals with sufficient power to accurately demodulate the information transmitted, but with no more power than is necessary. Accurately determining the amount of power that can be transmitted with an acceptable level of distortion allows the greatest transmit power range and the greatest data throughput.
In some systems, a closed loop power control system is used. Such closed loop power control systems require the receiver to provide feedback regarding the receiver's ability to accurately demodulate the information that is being transmitted. Such closed loop power control systems are relatively slow and require the receiver to demodulate the transmitted information before such feedback can be provided.
In addition to closed loop feedback control systems, transmitters are typically calibrated prior to being sent out to the field. The calibration is an attempt to determine the dynamic range of the transmitter and determine the particular output levels at which distortion will occur and to determine the amount of loss that the signal will incur prior to being transmitted.
FIG. 1 is a simplified block diagram of a transmission section 100 of a transmitter. The maximum power that a transmitter can transmit is typically determined at the time the circuit is designed. Once the circuit is manufactured, the circuit is tested to determine the characteristics of the particular components that comprise the transmission section 100. The transmission section 100 is connected to a test load that simulates the conditions under which the transmission section 100 will be operating when put to use in the field. The transmission section 100 is placed in calibration/test mode by the combination of a receive/transmit (R/T) switch 102 and a calibration switch 112. In calibration/test mode, the R/T switch 102 disconnects a power amplifier (PA) 104 from a filter 106. In addition, the calibration switch 112 is closed to connect the output of the PA 104 to the input of a detector 114. The output of the PA 104 is also connected to a nominal test load 116 having a resistance that is equal to the impedance of the medium into which the transmission is to be launched (e.g., 75 ohms). Therefore, when the switch 112 is closed, the amount of power output by the PA 104 is measured by the detector 114.
A test signal is injected into the PA 104. A measurement made by the detector 114 on the test signal is reported to a PA controller 118. The PA controller 118 adjusts the gain of the PA 104 until the signal level is at a target value. The target value is determined based upon an estimate of a set of factors. These factors include the errors that might be present in the detector 114, any uncertainty in the level at which the PA 104 will begin to compress/distort the signals being amplified and the amount of distortion and attenuation due to the output filter 106 and the impedance of the load (not shown) coupled to the output port 108. Once the proper gain is determined for the PA 104, the unit is ready to be used in the field.
Accounting for all of the uncertainties associated with factors noted above requires a relatively large safety margin. That is, the transmission section 100 may need to operate at 3 dB, or more, above what is determined by the test procedure to be at the point at which information delivered to the load can be accurately demodulated. Accordingly, the transmitter will deliver enough power to meet the desired target in the majority of cases, but with more power than might be appropriate for the average situation. This is done in order to ensure that the power settings will provide a reliable signal (acceptable receive power) in the majority of situations. Even transmitting with a safety margin of 3 dB (i.e., doubling the target level output power) may not be sufficient to ensure that the target level output power will actually be delivered in all cases. The margin must be selected such that when the unknown factors noted above are at their worst case values, every unit (or at least every unit minus a negligible number of outlier units) will be able to deliver a signal that has an acceptable power level without excessive distortion.
While the margin noted above is required in order to ensure proper operation in nearly all cases, transmitting with such a margin is not necessary in most cases. Transmitting with a margin that is greater than necessary in most cases means that the DC power is significantly higher than necessary in most cases (i.e., nearly double that required). Yet, the resulting operation may still be sub-optimal in outlying cases due to the uncertainties that exist from unit to unit and situation to situation. In order to optimize the power level of transmissions, extensive work is done in the laboratory to obtain statistics and determine the most efficient and effective margin possible. Even with such work, a compromise is required. That is, a balance must be struck between providing more power than is optimal in some cases under the assumption that the more power will be necessary than then is actually the case and transmitting with less power than is optimal in cases in which these factors require more power than is assumed.
In addition to the above situations, in systems that use a closed loop power control system, the path between a transmitter and one receiver may have more loss than the path to several other receivers. In this case, the transmitter will be driven to transmit more power than is required for the receivers communicating over the low loss paths. Transmitting at the higher power level can cause distortion in the transmitter that results in errors occurring in the information transmitted. Therefore, a relatively large number of receivers will suffer because of the needs of one receiver.
Accordingly, there is presently a need for a method and apparatus that can more efficiently and effectively determine the amount of power with which a transmitter should transmit signals to ensure that a target output power will be delivered with an acceptable level of distortion.